Electromechanical energy harvesting system

ABSTRACT

A method of electromechanical energy harvesting includes sensing electrical current generated by relative movement of a coil and a magnetic flux source produced by movement of a cantilevered beam in response to ambient vibration energy, determining a vibration characteristic of the ambient vibration energy, and adjusting an effective flexible length of the cantilevered beam as a function of the vibration characteristic and the sensed electrical current.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a division of U.S. patent application Ser. No.12/086,233, filed Jun. 9, 2008, and in turn claims priority to PCTApplication No. PCT US05/44799, filed Dec. 9, 2005.

BACKGROUND OF THE INVENTION

The present invention relates to generators, and more particularly to asystem for harvesting kinetic energy.

Wireless sensor/actuator/relay nodes can be powered by harvesting energyfrom a range of sources present in the environment. In this context,“harvesting” kinetic energy is the generation of electrical power fromambient energy that is present in the environment. Energy harvesting hasadvantages over systems that require fuels or energy cells (e.g.,batteries), as fuels and energy cells require labor to replenish orreplace as they are inevitably depleted. Systems for harvesting energyinclude, for example, solar cells, thermoelectric generators, kineticgenerators, radio wave powered systems, systems utilizing leakagemagnetic or electric fields, etc. In some sensor/actuator/relayapplications, the only source of available energy is ambient kineticenergy, for example: container security systems, animal tracking, andcondition monitoring of machine parts (e.g., motors, turbines, pumps,gearboxes) and inaccessible structures (e.g., bridges, roads).

The nature of a kinetic energy harvesting mechanism in a self-containedsystem depends upon the application. Kinetic energy harvesting devicescan be divided into two groups: (1) acceleration/vibration and springmass system devices, e.g., kinetic watches, vibration generators, movingmagnet linear generators, and (2) repeated straining physicaldeformation devices, e.g., piezoelectric generators and magnetic shapememory generators.

BRIEF SUMMARY OF THE INVENTION

A method of electromechanical energy harvesting according to the presentinvention includes sensing electrical current generated by relativemovement of a coil and a magnetic flux source produced by movement of acantilevered beam in response to ambient vibration energy, determining avibration characteristic of the ambient vibration energy, and adjustingan effective flexible length of the cantilevered beam as a function ofthe vibration characteristic and the sensed electrical current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of an energy harvesting system according to thepresent invention.

FIG. 2 is a top view of the energy harvesting system of FIG. 1.

FIG. 3 is an end view of the energy harvesting system of FIGS. 1 and 2.

FIG. 4A is a perspective view of a portion of an embodiment of an energyharvesting system having four permanent magnets.

FIG. 4B is a perspective view of a portion of another embodiment of anenergy harvesting system having two permanent magnets.

FIG. 5 is a side view of a self-adjustable energy harvesting systemaccording to the present invention.

FIG. 6 is a top view of the self-adjustable energy harvesting system ofFIG. 5.

FIG. 7 is a cross-sectional view of a portion of the self-adjustableenergy harvesting system of FIGS. 5 and 6, taken along line 7-7.

FIG. 8 is a block diagram illustrating a control system of an energyharvesting system according to the present invention.

FIG. 9 is a block diagram illustrating an alternative control system ofan energy harvesting system according to the present invention.

DETAILED DESCRIPTION

The present invention relates to an electromechanical energy harvestingdevice that can be used to convert ambient kinetic energy to electricalenergy. A magnetic flux source is supported at the free end of aspring-like cantilevered arm. The flux source can vibrate relative to astationary coil and thereby generate an electrical current in the coil.The effective flexible length of the cantilevered arm can be optimizedin relation to its natural or resonant frequency and the frequency ofthe (applied) ambient kinetic energy source, in order to improve thegeneration of electrical power.

FIGS. 1, 2 and 3 illustrate one embodiment of an energy harvestingdevice 20 according to the present invention. The device 20 includes abase 22, a cantilevered arm 24, a yoke 26, a magnetic flux-generatingassembly 28 that includes a plurality of permanent magnets (PMs) 28 aand 28 b, and a coil 30.

In the embodiment shown in FIGS. 1-3, the base 22 includes a lowerportion 32 and a side portion 34, which is substantially perpendicularto the lower portion 32. A number of holes 36 are disposed in the base22. The base 22 provides structural support for the device 20. The holes36 of the base 22 facilitate engagement or mounting of the device 20 ata desired location, for example, by providing screws or other types offasteners connected through the holes 36. In further embodiments, othertypes of connection features can be used as alternative to the holes 36.It should also be understood that in further embodiments, the base 22can have other configurations. The base 22 is preferably formed of anonmagnetic material.

The device 20 can be placed in a controlled air environment (e.g., in avacuum). For example, the device 20 can be placed in a hermeticallysealed enclosure (not shown). Controlling the air environment can reduceair friction that might otherwise undesirably limit movement ofcomponents of the device 20. Moreover, the controlled air environmentcan mitigate wear and damage to the device 20 (e.g., to help preventrusting or other oxidation).

The cantilevered arm 24, synonymously called the cantilevered beam orthe flat spring, has a fixed end 24 a and a free end 24 b. The fixed end24 a of the cantilevered arm 24 is attached to the side portion 34 ofthe base 22 with fasteners 38 (e.g., screws, adhesive, a mechanicalinterference connection, etc.). The cantilevered arm 24 has a width w, athickness t, and an effective flexible length L, which is defined as thelength along which the arm 24 can flex through a selected degree offreedom. In one embodiment, the cantilevered arm 24 can have thefollowing dimensions: the width w is about 2-20 mm, the thickness t isabout 0.05-0.6 mm, and the effective flexible length L is about 5-40 mm.The cantilevered arm 24 comprises a resilient material (e.g., resilientpolymers, metallic materials, etc.) that permits bending. For example,the cantilevered arm 24 can comprise spring steel or copper beryllium.

The cantilevered arm 24 is configured such that its thickness t issubstantially less than its width w and its length L. That configurationmeans that the cantilevered arm 24 has substantially only one degree offlexural freedom and can vibrate only perpendicularly to the surface ofthe lower portion 32 of the base 22 of the device 20, that is, parallelto a plane of the coil 30.

The yoke 26 is attached to the free end 24 b of the cantilevered arm 24with fasteners 40 (e.g., screws, adhesive, a mechanical interferenceconnection, etc.). In one embodiment, the yoke 26 has a U-shapedconfiguration with a pair of legs 26 a and 26 b extending from a baseportion 26 c. The yoke 26 can comprise a magnetic material (i.e., amaterial that conducts magnetic flux), for example, mild steel.

The magnetic flux-generating assembly 28 can comprise one or moreelements attached to the yoke 26, for instance, the assembly 28 cancomprise a pair of elements 28 a and 28 b attached to the legs 26 a and26 b of the yoke 26, respectively. The magnetic flux-generating assembly28 is in turn supported by the cantilevered arm 24. The magneticflux-generating assembly 28 can produce a strong magnetic flux densityin an air gap formed between poles of the opposed elements 28 a and 28b.

The magnetic flux-generating assembly 28 can comprise one or morerare-earth high energy permanent magnets (PMs), such as sintered NdFeB,and SmCo, or Alnico and Barium Ferrite PMs. According to the presentinvention, those PMs can have a number of alternative configurations.For example, FIG. 4A is a perspective view of a portion of an embodimentof an energy harvesting device 20 with a magnetic flux-generatingassembly 28 having four PMs 28 a′, 28 b′, 28 c′ and 28 d′. Using fourPMs (28 a′-28 d′), a magnetic circuit 41 a is formed such that magneticflux passes through the air gap twice from one PM to another PM ofdifferent polarity. That is, magnetic flux passes across the air gapbetween PMs 28 a′ and 28 b′ and across the air gap between PMs 28 c′ and28 d′. The flux circuit is completed between PMs 28 b′ and 28 c′ as wellas between 28 d′ and 28 a′ across portions of the legs 26 a and 26 b ofthe yoke 26. However, in this embodiment, the yoke 26 need not be madeof a magnetic material (i.e., a material that conducts magnetic flux).

FIG. 4B is a perspective view of a portion of another embodiment of anenergy harvesting device 20 with a magnetic flux-generating assembly 28having two PMs 28 a and 28 b (a similar embodiment is shown in FIGS.1-3). Using two PMs 28 a and 28 b, magnetic flux passes the air gap onlyonce and a ferromagnetic return path for the flux is required tocomplete a flux circuit 41 b. This return path is created by forming theyoke 26 of a magnetic material (e.g., mild steel). Where the yoke 26 isformed of a magnetic material, the mass of the vibrating systemgenerally increases and cantilevered arm 24 of greater stiffness may berequired for efficient performance.

Turning again to FIGS. 1-3, the coil 30 can be a stationary multi-turncoil, and can have a relatively flat, plate-like configuration. In oneembodiment, the coil 30 has a width of about 5-50 mm, a length of about5-50 mm and a thickness of about 0.5-4 mm. The coil 30 is positioned inthe magnetic field of the magnetic flux-generating assembly 28. The coil30 can be secured to the lower portion 32 of the base 22. Supports orbraces 42 can be provided to better secure the coil 30 to the base 22.In one embodiment, the coil 30 can comprise turns of copper insulatedwire (magnet wire).

The magnetic circuit formed with the magnetic flux-generating assembly28 is suspended at the free end 24 b of the cantilever arm 24. When thedevice 20 is placed on a shaking or vibrating body that provides anexternal source of vibration (i.e., ambient kinetic energy), the device20 is excited by the external energy of vibration. The cantilever beam24 (and the magnetic flux-generating assembly 28) has substantially onlyone degree of freedom, and can vibrate only perpendicularly to thesurface of the lower portion 32 of the base 22 of the device 20, thatis, parallel to the plane of the coil 30.

The coil 30 is positioned between the poles of the PMs 28 a and 28 b asthe magnetic flux-generating assembly 28 vibrates. A mass m representsthe mass of the magnetic flux-generating assembly 28 placed at the freeend 24 b of the cantilever arm 24. The cantilever arm 24 with PMs underthe effect of forced motion (i.e., induced movement due to ambientkinetic energy) acts as a single-degree-of-freedom vibrating system thatcan be described by the following differential equation, where F_(m)sin(ωt) is an external force with the amplitude F_(m) considered forsimplicity as changing sinusoidally with the time t, ω=2πf is theangular frequency, f is the frequency and c is the coefficient ofviscous damping:m{umlaut over (x)}+c{dot over (x)}+kx=F _(m) sin(ωt)

The external force F_(m) sin(ωt) can be produced by an ambient kineticenergy source, such as, for example, a vibrating body on which thedevice 20 is placed.

As the magnetic flux-generating assembly 28 supported by thecantilevered arm 24 vibrates and moves relative to the coil 30, magneticflux passes through the turns of the coil 30 and generates an electricalcurrent therein. The induced voltage E in the coil 30 is proportional tothe frequency f of the vibrating magnetic flux-generating assembly 28,the normal component (i.e., the component perpendicular to the plane ofthe coil) of the magnetic flux density B (see FIG. 3) produced by themagnetic flux-generating assembly 28, and the number of turns N of thecoil 30, such that:E∝fBN

If a load impedance Z_(L) (where Z_(L)=R_(L)+jX_(L)) is connected acrossterminals (not shown) of the coil 30, a closed electrical circuit iscreated. The rms current I in the electrical circuit and the rms voltageV across the load impedance Z_(L) are, respectively:

$I = \frac{E}{\sqrt{\left( {R_{c} + R_{L}} \right)^{2} + \left( {X_{c} + X_{L}} \right)^{2}}}$$V = {I\sqrt{\left( {R_{c}^{2} + X_{c}^{2}} \right.}}$where Z_(c)=R_(c)+jX_(c) is the internal impedance of the coil 30. Thecurrent I can then be rectified and used for charging an energy storagedevice such as a battery or a supercapacitor (not shown), which can, inturn, energize an electronic device (e.g., a wirelesssensing/actuating/relaying system). Usually, the induced voltage E isnot high enough to charge a battery, because about 2 Volts are generallyrequired by a diode rectifier to charge a battery. However, in furtherembodiments, the induced voltage E can be increased by using anelectronic quadrupler voltage rectifier or step-up transformer.

The moment of area (inertia) I_(a) and stiffness k of a cantilevered arm24 having a fixed end 24 a and an opposite free end 24 b are expressedby the following equations, where w is the width of the cantilever beam,t is its thickness, L is its effective flexible length, and E_(Y) is itsYoung's modulus (i.e., its modulus of elasticity):

$I_{a} = {\frac{1}{12}{wt}^{3}}$ $k = \frac{3E_{Y}I_{a}}{L^{3}}$

If the cantilevered arm 24 is loaded at its free end 24 b with a mass m,the natural frequency of the spring-mass system (i.e., the systemdefined substantially by the cantilevered beam 24 loaded with themagnetic flux-generating assembly 28) is:

$f_{nat} = {\sqrt{\frac{k}{m}} = {\frac{1}{2}\sqrt{\frac{E_{Y}{wt}^{3}}{{mL}^{3}}}}}$

Thus, the natural frequency f_(nat) of the spring-mass system, i.e.,cantilever beam 24 loaded at its free end 24 b with the mass m, can besimply controlled by changing the effective flexible length L of the arm24.

The energy of vibration is converted into electrical energy with highefficiency—approaching a relative maximum efficiency—if the frequency fof the external source of vibration is close to or equal to the naturalfrequency f_(nat) of the spring-mass system, as shown by the equationsabove. After solving the differential equation from above, the forcedvibration amplitude is:

$X_{m} = \frac{\frac{F_{m}}{k}}{\sqrt{\left\lbrack {1 - \left( \frac{f}{f_{nat}} \right)^{2}} \right\rbrack^{2} + \left\lbrack {2{\zeta\left( \frac{f}{f_{nat}} \right)}} \right\rbrack^{2}}}$and the magnification factor MF is:

${MF} = {\frac{X_{m}}{\frac{F_{m}}{k}} = \frac{1}{\sqrt{\left\lbrack {1 - \left( \frac{f}{f_{nat}} \right)^{2}} \right\rbrack^{2} + \left\lbrack {2{\zeta\left( \frac{f}{f_{nat}} \right)}} \right\rbrack^{2}}}}$where the damping ratio is ζ=c/c_(c)=c/(2mf_(nat)), c is the actualdamping coefficient and c_(c)=2mf_(nat) is the critical dampingcoefficient. The above function is maximized if:

$\left( \frac{f}{fnat} \right)^{2} = {1 - {2\zeta^{2}}}$

As seen by these equations, both the forced vibration amplitude X_(m)and the magnification factor MF take maximum values for a low dampingfactor ζ and a frequency of vibration f that is equal to or very closeto f_(nat).

In practice, the frequency of vibration f can change across a widerange, usually from a few up to hundreds of cycles per second (Hz). Thefrequency of vibration f is highly dependent on the particularapplication and the particular source of ambient kinetic energy. Often,the external force F_(m) sin(ωt) can be produced by a vibrating body onwhich the device 20 is placed. In this case, the differential equationgiven above is written as:m{umlaut over (x)}+c{dot over (x)}+kx=c{dot over (x)} _(b) +kx _(b)where x_(b) is the perpendicular vibration displacement at the base 22of the device 20.

Other kinds of displacements, such as in-plane rotation θ_(b) at thebase 22, can be translated to an equivalent perpendicular vibrationdisplacement at the base 22 according to the following equation:x _(b)(t)=L sin(θ_(b)(t))

Because the effect of rotations and displacements at the base 22 addlinearly, all displacements of base 22 can be translated to anequivalent perpendicular vibration displacement x_(b). The magnificationfactor MF is thus equivalently given by the following equation:

${MF} = {\frac{X_{m}}{\frac{F_{m}}{k}} = \frac{\sqrt{1 + \left\lbrack {2{\zeta\left( \frac{f}{f_{nat}} \right)}} \right\rbrack^{2}}}{\sqrt{\left\lbrack {1 - \left( \frac{f}{f_{nat}} \right)^{2}} \right\rbrack^{2} + \left\lbrack {2{\zeta\left( \frac{f}{f_{nat}} \right)}} \right\rbrack^{2}}}}$The magnification factor MF, according to the equation above, ismaximized if:

$\left( \frac{f}{f_{nat}} \right)^{2} = \frac{2}{\sqrt{1 + {8\zeta^{2}} + 1}}$Hence, the magnification factor MF again takes relative maximum valuesfor low damping factor ζ and frequency of vibration f equal to or veryclose to f_(nat).

The electromechanical energy harvesting device 20 increases efficiencyif the frequency of vibration f is close to the natural frequencyf_(nat) of the spring-mass system. Otherwise, the magnification factorMF is low, amplitude of vibration of the spring mass system X_(m) isalso low, and consequently the generated electric power may be less thandesired (e.g., it may not be sufficient to charge a battery). Thus, inone embodiment, the effective flexible length L of the cantilevered arm24 is selected such that the natural frequency f_(nat) of thespring-mass system is close to a frequency of vibration f (from anexternal vibration source). The value of length L can be selectedaccording to a predominant external vibration source, or otherwise, asdesired.

To approach maximum efficiency at all times, the natural frequencyf_(nat) of the mass-spring system should be dynamically adjusted to thefrequency f of the external source of vibration. This can beaccomplished by adjusting the effective flexible length L of thecantilever beam, because the natural frequency is inversely proportionalto L^(3/2). In general, adjustment of the effective flexible length L ofthe cantilever beam can be done by selectively positioning a stabilizeralong the cantilever beam using an electromechanical linear actuator andposition control.

FIGS. 5 and 6 illustrate a self-adjustable energy harvesting device 120.FIG. 7 is a cross-sectional view of a portion of the self-adjustableenergy harvesting device 120, taken along line 7-7 of FIG. 6. Withdevice 120, the coil 30, the magnetic flux-generating assembly 28, andthe cantilevered arm 24 are generally similar to those of the constantnatural frequency energy harvesting device 20 shown and described withrespect to FIGS. 1-3. The device 120 further includes a base 122, anactuator 150 and a stabilizer 152.

The base 122 has a lower portion 32 and a support 134 extendinggenerally perpendicularly from the lower portion 32. As seen in FIG. 7,the support 134 defines a first recess 134 a and a second recess 134 b.A fixed end 24 a of the cantilevered arm 24 is permanently fixed to thesupport 134 by fasteners 38.

The actuator 150 includes a linearly movable actuation piston 154. Theactuator 150 can be any type capable of producing or inducing a lineardisplacement. However, in many applications, a small size of theactuator and very low power consumption are important. In such asituation, it is preferable to use a linear actuator with a rotary motorand roller screw or ball lead screw, as opposed to a direct drive linearmotor. Although direct drive linear motors provide more accuratepositioning at higher speeds, they are characterized by much lower forcedensity than linear actuators with rotary motors and roller or ball leadscrews. Even a powerful PM brushless or stepping linear motor may lacksufficient electromagnetic force density. Also, a linear motor consumesmuch more electrical energy than a linear actuator with rotary motor androller or ball lead screw of a similar rating.

In the embodiment shown in FIGS. 5-7, the stabilizer 152 is a U-shapedmember having a pair of legs 152 a and 152 b and a base portion 152 c.The base portion 152 c of the stabilizer 152 is connected to theactuation piston 154 of the actuator 150. The legs 152 a and 152 b ofthe stabilizer 152 are disposed at opposite surfaces (i.e., at top andbottom surfaces) of the cantilevered arm 24. The stabilizer 152 canslidably move relative to the cantilevered arm 24 and, in particular, afirst end 152 d of the stabilizer 152 can be extended past the fixed end24 a of the cantilevered arm 24 and toward its free end 24 b. Apart fromvibrational movements, the cantilevered arm 24 is relatively stationarywhile only the stabilizer is pushed or pulled by the actuation piston154 of the actuator 150.

The cantilevered arm 24 extends into the first recess 134 a of thesupport 134. The second recess 134 b of the support 134 accepts thestabilizer 152, and serves as a guide for the stabilizer 152. Theposition of the stabilizer 152 relative to the cantilevered arm 24 isadjusted by the actuator 150. The natural frequency of the cantileveredarm 24 is controlled by moving the first end 152 d of the stabilizer 152closer or further from the free end 24 b of the cantilevered arm 24. Thestiffness of the stabilizer 152 (which can be selected as a function ofthe thickness of the stabilizer legs 152 a and 152 b) should be muchhigher than that of the cantilevered arm 24, otherwise the adjustment ofthe natural frequency would be weak.

As seen in FIG. 5, the effective flexible length L of the cantileveredarm 24, at any given time, is generally defined between the end portion26 c of the yoke 26 and the first end 152 d of the stabilizer 152.

FIG. 8 is a block diagram illustrating an example of a control systemfor an energy harvesting device (e.g., the device 120). The controlsystem includes an accelerometer 200 mounted on the cantilevered arm 24and a microprocessor circuit 202 having an analog/digital (A/D)converter 204, a comparator 206 and a servo amplifier 208. It should beunderstood that FIG. 8 is a simplified diagram, and other components maybe included that are not shown, for clarity.

The accelerometer 200 detects vibration of the energy harvesting device(e.g., the device 120) due to ambient energy and generates a frequencysignal, which is sent to the comparator 206. The frequency of thecurrent in the coil 30 is also sent as a signal to the comparator 206.The output frequency of the coil 30 is used as a feedback signal,following the induced voltage E in the coil 30. The comparator 206 iselectrically connected to the servo amplifier 208, which is electricallyconnected in turn to the actuator 150.

In operation, the frequency of the current in the coil 30 is used as afeedback signal that is compared, by the comparator 206, with thefrequency f of the source of vibration obtained from the accelerometer200. The comparator 206 generates a frequency error signal. Thefrequency error signal is amplified by the servo amplifier 208, and theoutput current of the servo amplifier 208 is sent to the actuator 150(i.e., the output current powers the stator windings of the actuator150). The following algorithm can be used for providing stabilizerposition control by regulating current i to the actuator 150:

$i = \left\{ \begin{matrix}{i_{\max},{{{{if}\mspace{14mu} f_{nat}} - f_{acc}} > \Delta}} \\{{\frac{i_{\max}}{\Delta}\left( {f_{nat} - f_{acc}} \right)},{{{if}\mspace{11mu}{{f_{nat} - f_{acc}}}} \leq \Delta}} \\{{- i_{\max}},{{{{if}\mspace{14mu} f_{nat}} - f_{acc}} < {- \Delta}}}\end{matrix} \right.$where i_(max) is the maximum operational current for the actuator 150,f_(nat) is the natural frequency of the spring-mass system, f_(acc) isthe main frequency component in the vibration (i.e., of the ambientkinetic energy source), and Δ is a constant that corresponds to the sizeof a minimum linear actuation interval for preventing excessiveswitching action (i.e., preventing excessive movement of the stabilizer152 by the actuator 150).

If the frequency of vibration f increases, the effective flexible lengthL of the cantilevered arm 24 should be reduced. If the frequency ofvibration f decreases, the effective flexible length L of thecantilevered arm 24 should be increased. In this embodiment, theactuator 150 does not need any encoder or other position sensor.

FIG. 9 is a block diagram illustrating an alternative control schemethat utilizes a frequency estimator. Electromechanical (EM) power 300 isgenerated by the device 120, and can be measured, for example, ascurrent in the coil 30.

The unknown frequency f of vibration of the cantilever arm 24 can beestimated by a frequency estimator 302 (e.g., a microprocessor) usingthe following equations:{umlaut over (z)}+2ζω_(f) ż+w _(f) ² z=ky{dot over (ω)}_(f) +g(ky−2ζω_(f) ż)z=0where z is a state of the estimator, y is an appropriate measurablysinusoidal output (e.g., current or power 300) and ω_(f) is the estimateof the frequency of y. Then, a length setpoint 304 for adjusting thestabilizer 152 relative to the cantilevered arm 24 can be determined.The effective flexible length L of the cantilevered arm 24 can beadjusted (i.e., tuned) to the estimated vibration frequency ω_(f) usingthe equation:

$L = {\frac{1}{2}\left( \frac{E_{y}{wt}^{3}}{2m\;{\omega_{f}}^{2}} \right)^{1/3}}$to generate the appropriate length setpoint 304 and to command theactuator 150 to move the stabilizer 152 accordingly.

In summary, the present invention describes a portable electromechanicalenergy harvesting device having a natural frequency of the cantileverbeam selected to equal or approximate a frequency of an externalvibration source. In one embodiment, the natural frequency of thecantilever beam is automatically continually adjustable to at leastapproximate a frequency of an external vibration source. Because therelative maximum generated energy occurs when mechanical resonanceoccurs, i.e. when the natural frequency of the cantilever beam-basedvibrating system is the same or close to the input frequency ofvibration for an external source, the self-adjustable device willapproach maximum efficiency. The natural frequency of the cantileverbeam is adjusted by changing the effective flexible length of thecantilever beam using a stabilizer positioned by an electromechanicalactuator and position control. Potential applications of a systemaccording to the present invention include: (1) wireless sensorsinstalled in security systems of containers or trailers, (2)condition-based monitoring of machinery and structures; (2) implantedmedical sensors; (3) wearable computers; (4) intelligent environments(e.g., “smart spaces”), etc.

It should be recognized that the present invention provides numerousbenefits, including: a simple construction; low cost due to reducednumber of parts, easy fabrication, and easy installation; highefficiency of conversion of vibrational energy into electrical energydue to adjustable length of a cantilever beam and adjustment of itsnatural frequency to the frequency of external vibration; no externalelectrical power and wiring is required; environmentally friendly designand operation; eliminates the need for slip rings and induction loops;maintenance free, as it does not require battery replacement; and highreliability.

Although the present invention has been described with reference toseveral alternative embodiments, workers skilled in the art willrecognize that changes may be made in form and detail without departingfrom the spirit and scope of the invention. For instance, the shape andpositioning of individual components of an energy harvesting systemaccording to the present invention can vary as desired. Moreover,different control schemes, control algorithms, and control circuits canbe used to adjust the effective flexible length of the cantilevered arm.

1. A method of electromechanical energy harvesting comprising: detectingelectrical current generated by relative movement of a coil and amagnetic flux source produced by movement of a cantilevered beam inresponse to ambient vibration energy; determining a vibrationcharacteristic of the ambient vibration energy; and adjusting aneffective flexible length of the cantilevered beam as a function of thevibration characteristic and the detected electrical current, whereinthe effective flexible length of the cantilevered beam is adjusted suchthat ambient vibration energy causes the cantilevered beam to vibrate atapproximately a resonant frequency.
 2. The method of claim 1, whereinthe effective flexible length is selected by adjustment of a stabilizerarm along the cantilevered beam.
 3. The method of claim 1, wherein thevibration characteristic is frequency of the ambient vibration energy.4. An electromechanical energy harvesting system comprising: acantilevered beam; a coil; a magnetic flux source, wherein the coil andthe magnetic flux source are configured to generate electrical currentby relative movement therebetween in response to ambient vibrationenergy that produces vibration of the cantilevered beam; a processorconfigured to determine a vibration characteristic of the ambientvibration energy; and a mechanism to adjust an effective flexible lengthof the cantilevered beam as a function of the vibration characteristicand the electrical current, wherein the processor provides a frequencyestimator and wherein the vibration characteristic is an estimatedvibration frequency of the cantilevered beam.
 5. The system of claim 4,wherein the mechanism is a stabilizer arm adjustably positioned alongthe cantilevered beam.
 6. The system of claim 5, wherein the stabilizerarm is U-shaped and extends along and opposite sides of the cantileveredbeam.
 7. The system of claim 4, wherein the vibration characteristic isfrequency of the ambient vibration energy.
 8. The system of claim 4,wherein the magnetic flux source is secured to a free end of thecantilevered beam.
 9. The system of claim 4, wherein the magnetic fluxsource comprises a plurality of permanent magnets.
 10. The system ofclaim 4 and further comprising: an accelerometer mounted to thecantilevered beam for sensing frequencies of the ambient vibrationenergy that produces vibration of the cantilevered beam.
 11. Anelectromechanical energy harvesting system comprising: a cantileveredbeam; a coil; a magnetic flux source, wherein the coil and the magneticflux source are configured to generate electrical current by relativemovement therebetween in response to ambient vibration energy thatproduces vibration of the cantilevered beam; a processor configured todetermine a vibration characteristic of the ambient vibration energy asa function of the electrical current; an accelerometer mounted to thecantilevered beam for detecting frequencies of the ambient vibrationenergy that produces vibration of the cantilevered beam; and a mechanismto adjust an effective flexible length of the cantilevered beam as afunction of the electrical current.
 12. The system of claim 11, whereinthe mechanism is further configured to adjust the effective flexiblelength of the cantilevered beam as a function of the vibrationcharacteristic in conjunction with the electrical current.
 13. Thesystem of claim 11, wherein the mechanism is a stabilizer arm adjustablypositioned along the cantilevered beam.
 14. The system of claim 13,wherein the stabilizer arm is U-shaped and extends along and oppositesides of the cantilevered beam.
 15. The system of claim 11, wherein thevibration characteristic is frequency of the ambient vibration energy.16. The system of claim 11, wherein the processor provides a frequencyestimator and wherein the vibration characteristic is an estimatedvibration frequency of the cantilevered beam.
 17. The system of claim11, wherein the magnetic flux source is secured to a free end of thecantilevered beam.